To still properly detect tire information such as pneumatic pressure information on a tire, even if the flat tire is repaired by using a flat tire repairing agent.

This transmission device used for a tire information monitoring system includes a sensor for detecting a state of gas filled in a tire cavity area, a transmitter, a casing provided with an air vent, and a tire valve integrated with the casing and fixed by being installed in the rim. An angle between a first straight line of extending the axis of the tire valve to the tire cavity area from an end part on the tire cavity area side and a second straight line of extending the axis of the tire valve extending toward the tire cavity area from an atmospheric side end part of the tire valve to the tire cavity area, is 20-80 degrees, and an angle between the second straight line and a third straight line of extending the axis of the air vent along the opening direction of the air vent, is 80-130 degrees, and a cutoff wall is arranged between the air vent and an air introducing part to the tire cavity area of the tire valve.